1. Field of the Invention
The present invention relates to a process for making rhenium-containing alloys by melting together the constituents that form the alloys, then casting the melt and allowing the melt to solidify. The present invention also concerns rhenium-containing alloys made according to such process.
The present invention relates, in particular, to a process for making an iron, cobalt or nickel base alloy, or an alloy containing a mixture of at least two of these base metals, wherein such alloy further contains rhenium; and the present invention is directed to an alloy made according to this process.
2. Background Information
In the context of the present specification, an iron, cobalt or nickel base alloy and an alloy containing a mixture of at least two of these base metals, wherein such alloy further contains rhenium, are understood to be alloys whose content of iron, cobalt and/or nickel is higher than that of rhenium and of any other constituent that may also be present in the alloys.
The superalloys, among other materials, belong to this category of alloys. According to Rompp Chemical Encyclopedia, 9th Edition, Stuttgart; New York; Georg Thieme Verlag, 1989 to 19921, 4393, these alloys have an extremely complex composition for use at very high temperatures. The base metal of the alloy is iron, nickel or cobalt, with admixtures of metals (cobalt, nickel, iron, chromium, molybdenum, tungsten, tantalum, niobium, aluminum, titanium, manganese and/or zirconium) and nonmetals (carbon and boron). Parts and components of superalloys are made by forming, casting or sintering, and derive their special properties from the precipitation or reaction kinetics of the elements involved, as a function of the manufacturing process and the applied temperature. Superalloys are used in the construction of engines and propulsion units, and are utilized in the fields of energy engineering and aerospace.
The choice of admixtures of elements is guided by the stresses and strains to which the superalloys are exposed during operation. For example, in DE 25 30 245 there is described a high-temperature-resisting, corrosion-resistant and oxidation-resistant superalloy which contains at least 50 vol % .gamma.'-phase and can comprise 14.3 wt % chromium, 13.5 wt % cobalt, 2.1 wt % titanium, 1.8 wt % aluminum, 9.2 wt % platinum and the rest nickel.
Superalloys for jet propulsion units may contain an admixture of metals including rhenium (Rompp Chemical Encyclopedia, 9th Edition, Stuttgart; New York; Georg Thieme Verlag, 1989 to 1992, 3867). Superalloys of this type comprise, for example, 10% cobalt, 8.7% tantalum, 5.9% tungsten, 5.7% aluminum, 5% chromium, 3% rhenium, 1.9% molybdenum, 0.1% hafnium and the rest nickel (EP 0 554 198) or 2% chromium, 3.7% cobalt, 32% molybdenum, 8.2% tantalum, 6.2% aluminum, 6.3% rhenium, 4% vanadium, 0.24% carbon and the rest nickel (Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Weinheim; VCH Verlagsgesellschaft mbH, 1985 to 1995, Volume A13, 61).
Alloys of iron, cobalt and nickel, which contain rhenium, are known. In German Patent 729 862, for example, there is described a material for making corrosion-resistant, naturally hard and abrasion-resistant articles of rhenium or of high-rhenium alloy containing admixtures of platinum metals, tungsten, chromium, molybdenum, iron, cobalt and nickel individually or in combination, in a proportion of 0.1 to 50%. In JP 52-52106, there is described an electrodeposited alloy of rhenium and cobalt or nickel for electrical contacts.
To make iron, cobalt and/or nickel base alloys containing rhenium, by melting together the constituents forming the alloys, and then casting the melt and allowing the melt to solidify, melts (premelts) are usually formed first from the main constituents in a vacuum induction-melting furnace. Then the admixtures are introduced into the melt, wherein the rhenium is used in the form of pellets obtained from rhenium powder by pressing and sintering (in a vacuum, or under a reducing atmosphere, usually hydrogen).
Because of some characteristic properties of rhenium, this manufacturing process is complex and is associated with problems that may impair the quality of the alloys.
The high melting point (3180.degree. C.) of rhenium, approximately 1700.degree. C. above that of the melt, and its high density (21.0 g/cm.sup.3 ; melt: about 8 g/cm.sup.3) make it difficult to melt rhenium and distribute it homogeneously in the melt.
Since rhenium is easily oxidized to rhenium heptoxide, Re.sub.2 O.sub.7, by atmospheric oxygen, any oxygen that, for example, still remains in the melt, despite thorough outgassing or is still present in the atmosphere of the melting furnace, may lead to the formation of rhenium heptoxide. Rhenium heptoxide sublimes at 250.degree. C. and above, so that the melt is undesirably depleted of rhenium and the alloys no longer satisfy the desired specifications for the same.